High energy rate processing of ferrous alloys in the metastable austenitic condition



Nov. 2, 1965 R. F. HARVEY 3,215,565

HIGH ENERGY RATE PROCESSING OF FERROUS ALLOYS IN THE METAS'I'ABLEAUSTENITIG CONDITION Filed D66. 24, 1964 START ar TRANsF'oRMATmN END OFQSFORMA APPLJGATIQN QF HIGH ENERGY R PROCEDURES H 2 no so so 2 5 IS 3060 2346810 203060 SECONDS M\NUTES Houns TIME.

IN V EN TOR.

United States Patent HIGH ENERGY RATE PROCESSING OF FERROUS ALLOYS INTHE METASTABLE AUSTENITIC CONDITION Richard F. Harvey, Ross Township,Allegheny County,

Pa. (214 Dombey Drive, Pittsburgh, Pa.) Filed Dec. 24, 1964, Ser. No.422,073 7 Claims. (Cl. 148-124) This application is acontinuation-in-part of my application Serial No. 95,977, filed March15, 1961, now abandoned, for High Energy Rate Forming and Heat TreatingFerrous Alloys in the Metastable Austenitic Condition.

The present invention relates to the application of high energy rateprocedures, such as high pressure by the initiation of explosion, toferrous alloys in the metastable austenitic condition. More particularlythis invention relates to the application of high energy rate procedureswhich involve the sudden or very rapid application of energy to ferrousalloys in the metastable austenitic; condition for hardening and/orforming. This invention further relates to a process for readilyobtaining higher deformations and strengths than the methods of theprior art.

High energy rate processes involve the very rapid or sudden release ofenergy or in other words involve the application of relatively highpressure at very high speed. Pressures up to about 4,000,000 lbs. persq. in. are obtainable with high explosives. Generally the deformationis accomplished in less than about 0.001 second and the speed of thematerial deformation is about 50 to 200 ft. per second as compared withonly about 1 to ft. per second for conventional mechanical processesincluding forging extruding, swaging, peening, drawing, etc. Suchconventional mechanical processes will be referred to as low velocityprocesses in the specification.

The displacement velocity is the actual velocity of the displaced metaland this should not be confused with the rate of energy release which inthe case of an explosion is referred to as the detonation velocity. Thismay be up to about 40,000 ft. per second for high explosives.

Also the critical impact velocity is an important consideration forforming because it limits the maximum strain rate at which materialexhibits some ductility.

For a further discussion of high energy rate processes reference is madeto the following: A Guide to the Literature on High VelocityMetalworking D.M.I.C. Report 179 Dec. 3, 1962, the Defense MetalsInformation Center, Battelle Memorial Institute, Columbus 1, Ohio.

This information submitted concerning the effects of high energy rateprocedures is known and is included to provide a better basis for theunderstanding of the present invention.

My investigations show that the application of high energy rateprocedures such as explosive shocking to the hardening and forming offerrous alloys in the metastable austenitic condition in accordance withthe teachings of this invention results in improved physical propertiestogether with excellent forming characteristics not obtainable by themethods of the prior art.

It is realized that the prior art discloses mechanical working ofmetastable austenite and many investigations are reported in theliterature starting with the work of the applicant which led to thegranting of US. Patent No. 2,717,846.

3,215,565 Patented Nov. 2, 1965 In a later investigation of workingmetastable austenite D. J. Schmatz et al. in US. Patent No. 2,934,463reports deformation over in the metastable austenitic condition.However, there are many difficulties in attempting severe deformationusing the conventional low velocity methods of working. For example, theequipment including bearings, dies, and machine components do not standup under such severe conditions and severe deformations cannot readilyand inexpensively be accomplished as a commercial practice with themethods of the prior art. The deficiencies and difiiculties withdeforming metastable austenite by the methods of the prior art arerecognized in a recent Department of Defense publication entitledSummary of Recommendations for Research and Development in Materials,July 1, 1961. This is available from the Office of Technical Services,Department of Commerce as P.B. 161,865. The report recognizes thepotential usefulness of thermal mechanical processing but recommendsfuture work in changes in the manner of working.

The difliculties encountered in attempting to obtain high deformationsis further exemplified by Technical Documentary Report No.ASD-TR-61-428, March 1962, by R. P. Sernka et a1. Fracturing of H 11occurred on open die forgineg H 11 steel in the metastable austeniticcondition with deformations over 45% and the maximum deformationobtainable for this grade was 73%.

The present invention is designed to meet the needs of defense andaerospace requirements for high strengths and it provides a new andimproved commercially feasible method of working metastable austenite toproduce unexpected and highly desirable results not disclosed by themethods of the prior art.

A principal objective of this invention is to provide a new method ofhardening and/ or forming ferrous metals in the metastable austeniticcondition which. is characterized by good formability and higherphysical properties than is obtainable by the methods of the prior art.

A further object of this invention is to provide a new method of deephardening which permits higher deforma tions than have heretofore beenobtainable. These high degrees of deformation can be readilyaccomplished by the teachings of this invention in commercial practicewithout undue damage to and problems with the deforming equipment whichis a charcteristic limitation of methods of the prior art.

A further object of this invention is to provide a new method ofhardening and forming where the critical impact velocity is high topermit greater ease of forming.

A further object of this invention is to provide a new method ofhardening and forming which results in high strength weight ratios.

Other objects will be apparent from the description which follows. Theapplication of high energy rate methods to ferrous alloys in themetastable austenitic condition results in excellent formability. Alsothis treatment results in greater conversion of metastable austenite tomartensite than would occur without the application of high pressures athigh speeds. A higher hardness of at least 2 points Rockwell C may beobtained on A.I.S.I. H 11 tool steel which is used rather widely foraircraft applications. Substantially all of the austenite is convertedto martensite on subsequent cooling to room temperature with the resultthat very excellent physical properties are obtained. The new formingand hardening technique is termed Blastforming.

This method of processing has since been investigated by others withhighly successful results. For example P. C. Johnson and B. C. Stern, ina paper delivered before the 92nd A.I.M.E. meeting in Dallas, Texas,reported a substantial improvement in the tensile and yield propertiesfor H 11 and D6AC steels which were explosively hardened inthesubcritical austenitic condition. Another recent investigation madeunder Air Force contract AF 33 (616) 8191 reported successful resultsfor steels explosively shocked in the metastable austenitic condition.

According to one method of applying the principles of the presentinvention, the forming and hardening of ferrous metal in the metastableaustenitic condition by high energy rate methods such as explosive shockmay be completed in four steps which are illustrated schematically withreference to the transformation curves in the drawing. The processing ofan air hardening tool steel commonly classified as A.I.S.I. H 11 isillustrated. H 11 is used for high temperature and aircraft applicationsand has the following typical analysis:

Percent Carbon 0.40 Manganese 0.40 Silicon 1.00 Chromium 5.00 Molybdenum1.40 Vanadium 0.50

Balance, substantially iron.

The transformation curves show the times required for the austenite tostart and to complete transformation at each temperature. Temperature inF. is plotted as the ordinate and time on a loragithmic scale is plottedas the abscissa.

The temperature at which martensite starts to form on cooling is knownas the M temperature and is about 520 F. for H 11 steel.

After the H 11 steel is heated above its critical temperature to renderit austenitic, it may then be cooled at a rate exceeding its criticalcooling rate to a temperature above the M temperature but below the noseof the transformation curve where pearlite starts to form isothermallyat about 1450 F.

In the so-called bay region above the M temperature of 520 F. for H 11steel and below the nose at about 1450 F the steel may be maintained forpredetermined lengths of time in the subcritical or metastableaustenitic condition. At 800 F., for example, the H 11 steel may be heldfor several hours before the austenite starts to transform isothermallyt Bainite.

In the preferred method of carrying out the principles of the presentinvention, four steps are involved which are illustrated schematicallyby lines 1-2, 2-3, 3-4, and 4-5 with relation to the transformationcurves in the drawing. Line 12 represents quenching the austenitizedsteel from above its critical temperature range at a rate at least equalto its critical cooling rate to a suitable temperature in the bay regionabove its M temperature. The line 2-3 represents holding the steel inthe subcritical, austenitic condition at about 800 F. for a length oftime to uniformly attain that temperature but insufficiently long topermit substantial transformation to Bainite. While it is preferable tohold the steel as represented by line 2-3 this is not essential andsatisfactory results are obtained by shocking the steel in themetastable austenitic condition on cooling through the metastableaustenitic range. Line 3-4 represents the application of high energyrate methodssuch as explosive shock to the steel in the metastableaustenitic condition. It will be understood that the application of highvelocity energy occurs very rapidly, generally in less than 0.001 secondand the length of line 3-4 in the drawing appears disproportionatelylong for purposes of illustration. Line 45 represents cooling the steelgenerally in air to room temperature.

Molten salts or hot oil may be used as the interrupted quenching mediumand these fluids may also be used as the transfer medium fortransmitting the shock energy to the steel in the metastable condition.

For high energy rate systems such as explosive and electrical dischargein liquids, the mass contribution is small and the velocity contributionis large towards kinetic energy.

For submerged salt, explosive processing, the weight of the moving saltfront is about 5 lbs. and the impact velocity is about 700 ft. persecond. Therefore the kinetic energy:- /2 (W/g)v /2 (5/32.2)'(7O0):37.980 ft. lbs.

The ratio of the velocity component (v to the mass component (W/g) isabout 3.2 10 By comparison it can be shown that for low velocityprocessing such as with a drop hammer this ratio is about 1.2.

C. W. Marschall in D.M.I.C. Report 192 entitled Hot- Cold Working ofSteel to Improve Strength, October 11, 1963, concludes that the degreeof strengthening in working steels in the metastable austeniticcondition is approximately proportional to the amount of deformation. Myinvestigations on explosively shocked H 11 steel in the subcriticalaustenitic condition using salt as the transfer medium indicate adeformation of Also the increase in hardness as a result of thisprocessing is 2 points Rockwell C.

A deformation of 95% is higher than the results of methods of the priorart and it should be emphasized that the present invention avoids themechanical difficulties to bearings, dies, mechanical components, etc.,when high deformation degrees are attempted with low velocity methods.It will be noted also there are no mechanical parts involved inexplosive shocking of steels in the metastable austenitic condition andthis factor may explain in part why higher deformations can be obtainedand why high degrees of deformation can be obtained relatively withoutdifiiculty.

While severe deformations have been reported with low velocity workingof metastable austenite, the problems involved in attempting highdeformations in commercial practice are formidable. The presentinvention eliminates a major difiiculty encountered with high degrees ofdeformation as attempted by the methods of the prior art and the presentinvention shows how very high degrees of deformation can be accomplishedreadily and without undue difiiculty. My investigations also show howhigher degrees of deformation can be accomplished than has previouslybeen obtained by the methods of the prior art.

Steels which have been worked in the metastable aus tenitic condition bylow velocity processing invariably show marked directional propertiesboth in microstructure and in physical properties.

Particularly it should be noted also that the highest deformationsreported for low velocity working of metastable austenite are forrolling where the elongation of working is substantially all in onedirection. As will be expected the structures and properties with thismethod of low velocity working are highly directional and therefore oflimited usefulness. My investigations of high velocity working ofmetastable austenite indicate that severe deformations can beaccomplished with greater uniformity of structure and without the markeddirectional characteristics of the practice of the prior art.

The occurrence of non-martensitic transformation products afterdeforming metastable austenite with low velocity methods results inlower physical properties and is undesirable. With severe deformationsthe metal may be heated by the energy of working to raise thetemperature to the zone where transformation to pearlite occurs. Highvelocity working in accordance with the teachings of this inventionminimizes this difficulty because the working is accomplished so rapidlythat there is insufiicient time for the heat-to be generated. Thisinvention makes it possible to process steels and ferrous alloys whichcould not be worked by slow velocity methods be= cause of relativelyshort incubation periods prior to transformation oi because heatgenerated in working results in undesirable, non-martensitictransformation products.

While I do not wish to be limited by the consequences of a theory andthere is always the likelihood that other factors may be advanced asadditional investigational work is conducted, I believe that higherdeformations are obtainable, readily and without difiiculty by highvelocity shocking steels in the subcritical austenitic condition becausethe critical impact velocity is raised substantially. The criticalimpact velocity limits the critical strain rate at which a metalexhibits some ductility and this value is below about 250 ft. per secondfor many annealed steels. Under conditions of high velocity shocking themetastable austenite has a higher critical impact velocity so thatformability is increased and high degrees of deformation areaccomplished with greater ease. During the high velocity processing ofmetastable austenite, it is thought also that the regeneration ofdislocations at a high rate may also be a factor.

A difiiculty with low velocity working of metastable austenite is thatwork hardening occurs and limits the amount of deformation which can beaccomplished. This is particularly true of forging. With high velocityprocessing in accordance with the principles of this invention, thedeformation of the metastable austenite is substantially completedbefore work hardening interferes.

Forming can be accomplished along with hardening. For example, a quenchhardenable steel tubing may be simultaneously hardened and formed byexpanding to the desired shape while confined in a die. Processing isdone in the metastable austenitic condition using high explosives whichdetonate to provide the desired shock wave. Molten salt at about 800 F.may be used in the bore as the transfer medium and the tubing is placedin a mold cavity so that the internal pressure expands the tubingagainst the mold walls to harden and shape the tubing. Hardening canalso be accomplished without forming as in the case of flat sheets of ahardenable alloy steel which may be subjected to an explosive charge inthe metastable austenitic condition using a flat die to support thesheets so that hardening occurs without forming.

For hardening by the principles of this invention, the ferrous alloyshould be capable of hardening on quenching and should be capable ofbeing rendered in the metastable austenitic condition. For forming it isnot necessary that hardening also occur and any ferrous alloy which canbe converted to the metastable austenitic condition is suitable for highvelocity forming in accordance with the teachings of this invention.

A deformation range of about 70% to 95% is applicable to the processingof ferrous metals in the metastable austenitic condition by highvelocity or high energy rate methods and preferably the degree ofdeformation should be over about 90%. While examples of explosiveshocking were cited, other high energy rate methods including electricaldischarge and pneumatic-mechanical means which result in a materialdeformation speed of about 50 to 200 ft. per second are alsosatisfactory. It will be understood that this invention may be otherwiseembodied within the scope of the following claims.

I claim:

1. The method of hardening a quench hardenable ferrous alloy by highvelocity deformation of metastable austenite, comprising the steps ofheating the ferrous a1- loy above its critical temperature to render itaustenitic, cooling the said ferrous alloy at a rate greater than itscritical cooling rate to a temperature range below the temperature ofpearlite formation but above the temperature of martensite formation,holding the said ferrous alloy in the stated temperature range for atime insufficient to form Bainite, subjecting the ferrous alloy toexplosive shocking to result in a displacement velocity of about 50 to200 ft. per second at a deformation range of about 70% to 95% andfinally cooling the said ferrous alloy to 'room temperature.

2. The improved method of hardening a quench hardenable ferrous alloy bydeformation of metastable austenite at high velocity which substantiallyavoids difficulties with dies and deforming equipment which constitute acharacteristic limitation of low velocity metal working methods, saidimproved method comprising the steps of heating the ferrous alloy aboveits critical temperature to render it austenitic, quenching at a rate inexcess of its critical cooling rate to render the said ferrous alloy inthe sub-critical, metastable austenitic condition, subjecting the saidferrous alloy to explosive shock ing to result in a displacementvelocity of about 50 to 200 ft. per second at a deformation range ofabout 70 to 95 and finally cooling the said ferrous alloy to roomtemperature.

3. The improved method of hardening a quench hardenable ferrous alloy bydeformation of metastable austenite at high velocities whichsubstantially avoids difiiculties with work hardening which interfereswith the accomplishment of high degrees of deformation with low velocityworking methods, said improved method comprising the steps of heatingthe ferrous alloy above its critical temperature to render itaustenitic, quenching at a rate in excess of its critical cooling rateto render the said ferrous alloy in the subcritical, metastableaustenitic condition, subjecting the said ferrous alloy to high velocitydeformation to result in a displacement velocity of about 50 to 200 ft.per second at a deformation range of about about 70% to 95% and finallycooling the said ferrous alloy to room temperature.

4. The improved method of harden-ing a quench hardenable ferrous alloyby deformation of metastable austenite at high velocities to minimizedirectional properties characteristic of severe deformations with lowvelocity metal working methods, said improved method comprising thesteps of heating the ferrous alloy above its critical temperature torender it austenitic, quenching at a rate in excess of its criticalcooling rate to render the said ferrous alloy in the metastableaustenitic condi-'' tion, subjecting the said ferrous alloy to explosiveshocking to result in a displacement velocity of about 50 to 200 ft. persecond at a deformation in excess of about and finally cooling the saidferrous alloy to room temperature.

5. The improved method of forming ferrous alloys in the metastableaustenitic condition at high velocities which comprises the steps ofheating the said ferrous alloy above its critical temperature to renderit austenitic, cooling the said ferrous alloy at a rate greater than itscritical cooling rate to a temperature range below the nose of thetransformation curve but above the M temperature to render it in themetastable austenitic condition, subjecting the said ferrous alloy tohigh energy rate shock at displacement velocities of about 50 to 200 ft.per second to form the said ferrous alloy to the desired shape andfinally cooling the said ferrous alloy to room temperature.

6. The improved method of forming a ferrous alloy in the metastableaustenitic condition at high velocities which comprises the steps ofheating the said ferrous alloy above its critical temperature to renderit austenitic,

cooling the said ferrous alloy at a rate in excess of its criticalcooling rate to render it in the subcritical, metastable austentiticcondition, subjecting the said ferrous alloy to explosive shock atpressures up to 4,000,000 lbs. per square inch in less than about 0.001second to form the said ferrous alloy to the desired shape, and finallycooling the said ferrous alloy to room temperature, said explosive shockresulting in a critical impact velocity which is substantially higherthan the critical impact velocity characteristic of the same ferrousalloy sub jected to low velocity working in the metastable austeniticcondition.

7. The improved method of hardening steel by severely 7 deformingmetastable austenite ath igh velocities to minimize the formation ofundesirable, non-martensitic transformation products which commonlyoccur With metastable austenite severely deformed by low velocitymethods, said improved method comprising the steps of first heating thesaid steel to above its critical temperature to render it austenitic,cooling at a rate in excess of its critical cooling rate to render it inthe subcritica'l, metastable austenitic condition, subjecting the saidsteel to explosive shock to result in a displacement velocity of about50 to 200 ft. per second in the said steel in the metastable austeniticcondition and finally cooling the said steel to room temperature.

References Cited by the Examiner UNITED STATES PATENTS 9/55 Harvey148-12.4 4/60 Schmatz et al. 148-12.4

OTHER REFERENCES DAVID L. RECK, Primary Examiner.

1. THE METHOD OF HARDENING A QUENCH HARDENABLE FERROUS ALLOY BY HIGHVELOCITY DEFORMATION OF METASTABLE AUSTENITE, COMPRISING THE STEPS OFHEATING THE FERROUS ALLOY ABOVE ITS CRITICAL TEMPERATURE TO RENDER ITAUSTENITIC, COOLING THE SAID FERROUS ALLOY AT A RATE GREATER THAN ITSCRITICAL COOLING RATE TO A TEMPERATURE RANGE BELOW THE TEMPERATURE OFPEARLITE FORMATION BUT ABOVE THE TEMPERATURE OF MARTENSITE FORMATION,HOLDING THE SAID FERROUS ALLOY IN THE STATED TEMPERATURE RANGE FOR ATIME INSUFFICIENT TO FORM BAINITE, SUBJECTING THE FERROUS ALLOY TOEXPLOSIVE SHOCKING TO RESULT IN A DISPLACEMENT VELOCITY OF ABOUT 50 TO200 FT. PER SECOND AT A DEFORMATION RANGE OF ABOUT 70% TO 95% ANDFINALLY COOLING THE SAID FERROUS ALLOY TO ROOM TEMPERATURE.